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Glossary

Aeration

The mixing of air and water, usually by bubbling air through water or by contact of water with air.

Balancing guide

The elevation that is used to help maintain a relative balance among tributary storage reservoirs. When water is drawn from tributary reservoirs, the balancing guide ensures that water is drawn equitably from each one, leaving the reservoirs at a similar position between the flood guide and the balancing guide.

CFS

Cubic feet per second, typically used as a measure of flow in a stream. A cubic foot is equivalent to about 7.5 gallons. A measure of 1,000 cfs is equal to about 7,500 gallons of water per second.

Commercial waterway

A marked nine-foot-draft navigation channel suitable for barge transportation on the Tennessee River and its tributaries.

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Day-Second-Feet

A measure of the volume of water accumulated at a flow rate of one cubic foot per second running for one day. A storage tank with a capacity of about 650,000 gallons would be required to store one day-second-foot of water.

Dissolved oxygen

The oxygen dissolved in water that is necessary to sustain aquatic life. It is usually measured in milligrams per liter (mg/L) or parts per million (ppm).

Downstream

In the direction of a stream's current

Drawdown

The process of lowering the water level in a reservoir. Drawdown is usually measured in feet.

Fill period

The spring period of lessening runoff, when reservoirs are filled at a rate designed to maintain flood storage and reach targeted summer pool elevations

Flood storage

The volume, or capacity, in a reservoir that is reserved for the storage of floodwater.

Flood guide level

The elevation of a reservoir above which the space is reserved for temporary storage of water to help reduce downstream flooding.

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Groundwater

Water that is located under the surface of the earth.

Headwater

The upstream portion of a watershed.

Headwater elevation

The water elevation immediately upstream of a dam.

Hydrology

The study of the distribution and movement of water.

Interbasin transfer

The transfer of water out of one river basin for the benefit of a public water system in another river basin. In 2005, the 11 interbasin transfers from the Tennessee River watershed diverted 13.6 million gallons per day.

Load

The amount of electric power that is drawn from TVA’s electric system at a given point in time.

Lock

An enclosed dam chamber with gates at each end that allow water to be admitted and released. The resulting change in water level allows vessels to be raised and lowered so they can pass over non-navigable parts of a river. The locks on the Tennessee River make navigation possible for 652 miles, from Knoxville, Tennessee, to Paducah, Kentucky.

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Main-river, or main-stem, reservoirs

Reservoirs where the seasonal lowering to provide storage for flood damage reduction is typically less than seven feet. Main-river reservoirs include (upstream to downstream) Fort Loudoun, Watts Bar, Chickamauga, Nickajack, Guntersville, Wheeler, Wilson, Pickwick, and Kentucky. They were designed to serve multiple purposes, especially commercial navigation and hydropower production.

Megawatt-hour (MWh)

An amount of energy that equals one million watt-hours. It is a common measure for electricity use over time.

Minimum flow

A release from one or more dams provided to meet hydropower production, reservoir level targets, downstream water needs (such as aquatic habitat, water supply, and waste assimilation), and other commitments. Project minimum flows are those required to be released from a specific dam over a specific time period — not the lowest amount of water that TVA can pass through a dam. System minimum flows are those needed at some point in the system to meet specific needs for power, waste assimilation, navigation, and other beneficial uses.

Minimum operations guide

The system minimum operations guide is a seasonal guide based on the sum of the storage in 10 tributary projects. It is used to determine what level of flow should be released from these projects when additional flow is required to meet downstream needs.

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Normal operating zone

The elevation range within which main-river reservoirs are operated for power production and summer mosquito control. During high-flow periods, the top of the normal operating zone may be exceeded for the regulation of flood flows.

Operating guidelines

A set of guidelines that includes flood guides, minimum flow requirements, water release requirements, and other requirements to meet system operating objectives.

Reservoir (pool) level

The elevation of the water in a reservoir at a given time, measured in feet above sea level.

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Reservoir Operations Study

A study carried out by TVA to determine whether changes in its reservoir operating policies would produce greater overall public value. TVA sought input from stakeholders, identified issues of importance, compared alternative operating policies, and provided opportunities for public review and comment. The TVA Board adopted the recommendations of the ROS in May 2004.

Restricted drawdown

A lowering of reservoir pool levels that is limited by one or more restrictions on the rate of change.

Run-of-river reservoir

A project that relies on the flow of a stream or river to produce hydropower but has little or no capacity to store water. (It is one of two major categories of projects, the other being storage.) Run-of-river projects pass water through a dam at nearly the same rate it enters the reservoir, so they have minimal seasonal changes in water levels.

Runoff

That portion of total rainfall that eventually enters a river. About 44 percent of rainfall in the drainage area of the Tennessee River system becomes runoff.

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Sluiceway

Vertical slots, generally located near the bottom of a dam, which can be opened to release water from the upstream reservoir without passing it through the turbines. When the water at a dam is too low to reach the spillway, the sluiceway can be used to release water faster than it can flow through the turbines.

Spillway

A channel or passageway around or over a dam through which water is released, or “spilled,” past the dam without going through the turbines. Spillways at some dams are controlled with gates. At others, water flows over the top of the spillway automatically when the reservoir level gets to a certain elevation. A spillway is a safety valve for a dam; it can be used to discharge rainfall and runoff from major storms as necessary to maintain the reservoir below a predetermined maximum level.

Storage reservoir

A reservoir that can be used to store water for a variety of purposes, such as flood damage reduction, power production, navigation, water supply, and recreation.

Tailwater

The part of a river downstream from a dam, where the flow and quality of the water are substantially affected by the dam discharge.

Temperature stratification

The variation of water temperature at different depths in a reservoir. The coldest water is typically the densest and is found at the bottom of the reservoir; the warmest water is at the surface. In the Tennessee Valley, reservoirs usually begin the process of stratification in spring and become highly stratified by May or June. The effect disappears by winter.

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Tennessee River system

The Tennessee River and its tributaries, which comprise a drainage area of about 41,000 square miles. It includes 125 counties that cover much of Tennessee and parts of Alabama, Kentucky, Georgia, Mississippi, North Carolina, and Virginia.

Thermal plant

A power plant that produces electricity from heat energy released by the combustion of a fossil fuel (coal, oil, or gas) or the consumption of a fissionable material (nuclear fuel).

Top of gates

The maximum controlled elevation at a project, typically the top of a spillway gate in a closed position or crest elevation of an uncontrolled outlet structure.

Tributary

A river or stream flowing into a larger stream. In this case it refers to the streams and rivers that eventually flow into the Tennessee River.

Tributary storage reservoirs

Reservoirs located on tributaries of the Tennessee River whose operating policies include a provision for the reservation of flood storage. They include Norris, Douglas, South Holston, Boone, Cherokee, Watauga, Fontana, Tellico, Chatuge, Nottely, Hiwassee, Blue Ridge, Tims Ford, Normandy, Bear Creek, Little Bear Creek, and Cedar Creek.

Unrestricted drawdown

The lowering of reservoir levels with no restrictions on the rate of change.

Upstream

In a direction opposite to or against the stream's current.

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